With reductions in crude protein (CP) levels and higher supplementation of crystalline essential amino acids (EAA) in swine diets, the supply of non-essential amino acids (NEAA), or nitrogen (N) required for the synthesis of NEAA, is also reduced increasing the ratio between EAA-N and total N in the diet. When diets are deficient in NEAA-N, non-protein N (NPN) could supply additional N required for the endogenous synthesis of NEAA. The main objective of the present thesis was to assess the efficiency of ammonia for providing extra N when diets are deficient in NEAA-N. Secondary objectives were to determine the effects of ammonia supplementation on the amino acid (AA) profile of retained protein as an indicator of AA requirements and quantification of ammonia absorption and metabolism in the portal-drained viscera (PDV) and liver. Added ammonia to a NEAA-N deficient diet increased body weight (BW) gain and N retention and rendered similar efficiencies as supplemented Glu and a mix of NEAA indicating that the capacity for NEAA synthesis was not limiting. Moreover, ammonia did not modify (P > 0.05) the AA profile of retained protein compared to the NEAA mix implying that feeding ammonia, and consequently increasing the endogenous synthesis of NEAA, does not alter utilization of EAA for protein deposition, maintaining estimated requirements for EAA in growing pigs. When added in the diets, ammonia was rapidly absorbed by the PDV with the peak of absorption within 15 min after feed intake. Recovery of ammonia in the portal vein only accounted for 53% of added ammonia without increasing urea net flux, implying ammonia utilization in the PDV. Across splanchnic organs, appearance of Ala, Glu and citrulline (Cit) increased with ammonia supplementation. The results demonstrate that dietary ammonia is an efficient N source when diets are deficient in NEAA-N, and that the capacity for de novo synthesis of NEAA is not limiting under these circumtances. The latter should be considered when estimating minimal inclusion levels of NEAA in the diet.